1. Field of the Invention
Aspects of the present invention relate to a heat exchanger for a fuel cell, and more particularly, to a heat exchanger that can automatically control the level of cooling water according to the heat generation of a fuel cell.
2. Description of the Related Art
A fuel cell is an apparatus that transforms chemical energy of a fuel directly into electrical energy through a chemical reaction. Thus, the fuel cell is a kind of electric generator that can generate electricity as long as a fuel is supplied.
FIG. 1 is a schematic drawing showing the principle of generating electricity from a conventional fuel cell. Referring to FIG. 1, electricity is generated by a reverse reaction of the electrolysis of water taking place through an electrolyte membrane 2 when air, which contains oxygen, is supplied to a cathode 1 and a fuel containing hydrogen is supplied to an anode 3. However, a voltage of the electricity generated from a unit cell 4 is not usually high enough to be used. Therefore, as depicted in FIG. 2, a plurality of unit cells 4 is connected in series in a stack 10.
During the electrochemical reaction, not only electricity but also heat is generated. Therefore, to maintain a smooth operation of the fuel cell, the heat must be continuously dissipated. Accordingly, a heat exchanger 20, as depicted in FIG. 2, is provided in the fuel cell. Referring to FIG. 2, flow channels 4a for passing cooling water for exchanging heat are formed in each unit cell 4 of the fuel cell. The cooling water absorbs heat from the stack 10 while passing through the flow channels 4a. The cooling water that absorbs the heat is cooled down by secondary cooling water in the heat exchanger 20, and is re-circulated through the flow channels 4a of the stack 10. At this time, the circulation of the cooling water is not achieved by an additional force, but by natural convection of water, that is, by overflow of boiling water due to the heat absorbed from the surroundings. For example, as depicted in FIG. 3, when the cooling water is filled in the flow channels 4a at an appropriate level, the cooling water starts absorbing heat, and as a result, the cooling water starts boiling and overflows. The cooling water that overflows enters the heat exchanger 20 and is cooled down by secondary cooling water. Afterward, the cooled cooling water is re-circulated in the stack 10.
However, in many cases, the fuel cell is operated at a load smaller than a designed load. This is called a partial load operation state wherein the fuel cell generates less power than the designed power. In this state, the amount of heat generated is reduced, and thus, the amount of heat absorbed by the cooling water is also reduced. Thus, the convection of the cooling water is not achieved properly since the cooling water does not boil enough to overflow. In the related art, to solve this problem, the temperature of the cooling water entering the heat exchanger 20 is measured using a thermo-sensor 30. If the temperature of the cooling water is lower than a designed value, that is, the fuel cell operates in a partial load operation state, the cooling operation is not performed until the temperature of the cooling water in the stack 10 rises enough so that the cooling water can be circulated by closing a solenoid valve 40 installed at a cooling water outlet of the heat exchanger 20.
However, in this method, the cooling water does not overflow but fluctuates, that is, the water level in the stack 10 goes up and down until the temperature of the cooling water rises enough to be circulated. Thus, some regions of the flow channels 4a do not come in to contact with the cooling water for a period of time. As a result, these regions are exposed to superheated steam for a period of time, and thus the thermal resistance of the stack 10 is greatly reduced.
Also, this system requires complicated control devices such as the thermo-sensor 30 for measuring the temperature, the solenoid valve 40 for closing and opening the flow channels 4a, and a controller (not shown) for controlling these elements.